Temporal range: Late Triassic - Early Jurassic, 220–195Ma
|Upper and lower molars of Kuehneotherium|
D.M. Kermack et al., 1968
Kuehneotherium is an early mammaliaform genus that lived during the late Triassic period and is characterized by reversed-triangle pattern of molar cusps. Although a large number of fossils have been found, the fossils are limited to teeth, dental fragments, and mandible fragments. The genus includes Kuehneotherium praecursoris and all related species. It was first named and described by Doris M. Kermack, K. A. Kermack, and Frances Mussett in November 1967. The family Kuehneotheriidae and the genus Kuehneotherium were created to house the single species Kuehneotherium praecursoris. Modeling based upon a comparison of the Kuehneotherium jaw with other mammals indicates they were about the size of a modern day shrew between 4 and 5.5 g at adulthood.
Kuehneotherium is thought to be an insectivore that could consume only soft-bodied insects such as moths. Their teeth were shaped for vertical shearing and could not crush harder prey. They lived alongside another early mammal, Morganucodon, which had teeth that could crush harder insects such as beetles. This distinction in diet shows that early mammals adapted to have a separate feeding niche so they would not compete for food.
Geographic distribution and geological age
Fossil teeth of the first identified Kuehneotherium preacursoris specimens were found in the Pontalun Quarry in a single fissure pocket in South Wales. The deposit found in limestone is from the Late Triassic. (Whiteside and Marshall 2008) Additional Kuehneotherium fossils have been found in rock formations of the Middle Jurassic of Britain (Somerset), Luxemburg, the Late Triassic of France (Saint-Nicolas-de-Porte), the Lower Jurassic Kota Formation of India, and the Late Triassic of Greenland.
During the Late Triassic Period the supercontinent Pangaea was intact, allowing easy interchange and migration of mammals across the connected continents. This explains the wide distribution of Kuehneotherium fossils found throughout Greenland, Europe, and India. When the continents began to rift apart during the Jurassic, shallow seas covered the British Isles, where Kuehneotherium was first found. Mammal remains were swept into limestone caves and fissures formed by the shallow seas and were preserved as fossils in clastic sediment.
The climate Kuehneotherium lived in was hot and dry during this part of the early Mesozoic. Conifer plants thrived and spread throughout Pangaea. As the continents rifted apart during the Early Jurassic the climate was more humid. Ferns, horsetails, cycads, and mosses were common in both the Triassic and Jurassic, however they were more prevalent in the more humid Jurassic Period.
The phylogenetic position for Kuehneotherium has been widely debated. Kuehneotherium was once classified as a therian mammal (the common ancestor of marsupials, placentals, and their descendants). However, additional fossils of basal mammals have been found that predate the Kuehneotherium on the geological timescale and the relationships of early mammals were re-evaluated. Kuehneotherium is now placed in the more basal clade called Holotheria. Kuehneotherium preacursoris is the earliest mammal categorized as holotherian. Holotheria includes species in which the main and accessory molar cusps are arranged in a triangle. Kuehneotherium’s place in Holotheria is considered unstable, as it is difficult to determine a species characteristics based upon mandible and dental fragments.
Significance to the evolution of mammalian dentition
Study of the initial development of molar cusp triangulation in Kuehneotherium preacursoris was key in the early understanding of the transition between triconodont and crown therian molars. citation
Kuehneotherium dentition shows a significant link between mammaliaform triconodont shaped teeth used for in a puncture-crushing pattern, to modern crown therian molars that chew vertically and chew horizontally. Therian mammals such as marsupials and placentals shared a common ancestor that characterized by an upper molar with three main cusps arranged in a triangle that fits into the lower molar that has a reversed triangle and basin-like heel. Later discovery of the earlier more basal mammal Woutersia, provided additional information on this dental transition. Kuehneotherium, like other mammals had 2 sets of teeth during its life. It is speculated that they may have had up to 6 lower molars with the last molar being added to the back later in life. The evidence for this is that the post canine tooth row shifts backwards as the animal grew. They had 5-6 premolars; the first four premolars are single rooted. Anterior premolars would have been shed in late adulthood and not replaced. The lower jaw is a more basal morphology with a prominent postdentary groove where more developed postdentary bones would attach. The enamel microstructures of Kuehneotherium teeth were synapsid columnar enamel characterized by a pattern of columnar, prism-less structures.
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